RFC 4213 Basic IPv6 Transition Mechanisms October 2005
- Configured tunneling of IPv6 over IPv4: A technique for
establishing point-to-point tunnels by encapsulating IPv6 packets
within IPv4 headers to carry them over IPv4 routing
infrastructures.
The mechanisms defined here are intended to be the core of a
"transition toolbox" -- a growing collection of techniques that
implementations and users may employ to ease the transition. The
tools may be used as needed. Implementations and sites decide which
techniques are appropriate to their specific needs.
This document defines the basic set of transition mechanisms, but
these are not the only tools available. Additional transition and
compatibility mechanisms are specified in other documents.
1.1. Terminology
The following terms are used in this document:
Types of Nodes
IPv4-only node:
A host or router that implements only IPv4. An IPv4-only node
does not understand IPv6. The installed base of IPv4 hosts and
routers existing before the transition begins are IPv4-only
nodes.
IPv6/IPv4 node:
A host or router that implements both IPv4 and IPv6.
IPv6-only node:
A host or router that implements IPv6 and does not implement
IPv4. The operation of IPv6-only nodes is not addressed in
this memo.
IPv6 node:
Any host or router that implements IPv6. IPv6/IPv4 and IPv6-
only nodes are both IPv6 nodes.
IPv4 node:
Any host or router that implements IPv4. IPv6/IPv4 and IPv4-
only nodes are both IPv4 nodes.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
Techniques Used in the Transition
IPv6-over-IPv4 tunneling:
The technique of encapsulating IPv6 packets within IPv4 so that
they can be carried across IPv4 routing infrastructures.
Configured tunneling:
IPv6-over-IPv4 tunneling where the IPv4 tunnel endpoint
address(es) are determined by configuration information on
tunnel endpoints. All tunnels are assumed to be bidirectional.
The tunnel provides a (virtual) point-to-point link to the IPv6
layer, using the configured IPv4 addresses as the lower-layer
endpoint addresses.
Other transition mechanisms, including other tunneling mechanisms,
are outside the scope of this document.
The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
2. Dual IP Layer Operation
The most straightforward way for IPv6 nodes to remain compatible with
IPv4-only nodes is by providing a complete IPv4 implementation. IPv6
nodes that provide complete IPv4 and IPv6 implementations are called
"IPv6/IPv4 nodes". IPv6/IPv4 nodes have the ability to send and
receive both IPv4 and IPv6 packets. They can directly interoperate
with IPv4 nodes using IPv4 packets, and also directly interoperate
with IPv6 nodes using IPv6 packets.
Even though a node may be equipped to support both protocols, one or
the other stack may be disabled for operational reasons. Here we use
a rather loose notion of "stack". A stack being enabled has IP
addresses assigned, but whether or not any particular application is
available on the stacks is explicitly not defined. Thus, IPv6/IPv4
nodes may be operated in one of three modes:
- With their IPv4 stack enabled and their IPv6 stack disabled.
- With their IPv6 stack enabled and their IPv4 stack disabled.
- With both stacks enabled.
IPv6/IPv4 nodes with their IPv6 stack disabled will operate like
IPv4-only nodes. Similarly, IPv6/IPv4 nodes with their IPv4 stacks
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
disabled will operate like IPv6-only nodes. IPv6/IPv4 nodes MAY
provide a configuration switch to disable either their IPv4 or IPv6
stack.
The configured tunneling technique, which is described in Section 3,
may or may not be used in addition to the dual IP layer operation.
2.1. Address Configuration
Because the nodes support both protocols, IPv6/IPv4 nodes may be
configured with both IPv4 and IPv6 addresses. IPv6/IPv4 nodes use
IPv4 mechanisms (e.g., DHCP) to acquire their IPv4 addresses, and
IPv6 protocol mechanisms (e.g., stateless address autoconfiguration
[RFC2462] and/or DHCPv6) to acquire their IPv6 addresses.
2.2. DNS
The Domain Naming System (DNS) is used in both IPv4 and IPv6 to map
between hostnames and IP addresses. A new resource record type named
"AAAA" has been defined for IPv6 addresses [RFC3596]. Since
IPv6/IPv4 nodes must be able to interoperate directly with both IPv4
and IPv6 nodes, they must provide resolver libraries capable of
dealing with IPv4 "A" records as well as IPv6 "AAAA" records. Note
that the lookup of A versus AAAA records is independent of whether
the DNS packets are carried in IPv4 or IPv6 packets and that there is
no assumption that the DNS servers know the IPv4/IPv6 capabilities of
the requesting node.
The issues and operational guidelines for using IPv6 with DNS are
described at more length in other documents, e.g., [DNSOPV6].
DNS resolver libraries on IPv6/IPv4 nodes MUST be capable of handling
both AAAA and A records. However, when a query locates an AAAA
record holding an IPv6 address, and an A record holding an IPv4
address, the resolver library MAY order the results returned to the
application in order to influence the version of IP packets used to
communicate with that specific node -- IPv6 first, or IPv4 first.
The applications SHOULD be able to specify whether they want IPv4,
IPv6, or both records [RFC3493]. That defines which address families
the resolver looks up. If there is not an application choice, or if
the application has requested both, the resolver library MUST NOT
filter out any records.
Since most applications try the addresses in the order they are
returned by the resolver, this can affect the IP version "preference"
of applications.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
The actual ordering mechanisms are out of scope of this memo.
Address selection is described at more length in [RFC3484].
3. Configured Tunneling Mechanisms
In most deployment scenarios, the IPv6 routing infrastructure will be
built up over time. While the IPv6 infrastructure is being deployed,
the existing IPv4 routing infrastructure can remain functional and
can be used to carry IPv6 traffic. Tunneling provides a way to
utilize an existing IPv4 routing infrastructure to carry IPv6
traffic.
IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
IPv4 routing topology by encapsulating them within IPv4 packets.
Tunneling can be used in a variety of ways:
- Router-to-Router. IPv6/IPv4 routers interconnected by an IPv4
infrastructure can tunnel IPv6 packets between themselves. In
this case, the tunnel spans one segment of the end-to-end path
that the IPv6 packet takes.
- Host-to-Router. IPv6/IPv4 hosts can tunnel IPv6 packets to an
intermediary IPv6/IPv4 router that is reachable via an IPv4
infrastructure. This type of tunnel spans the first segment of
the packet's end-to-end path.
- Host-to-Host. IPv6/IPv4 hosts that are interconnected by an IPv4
infrastructure can tunnel IPv6 packets between themselves. In
this case, the tunnel spans the entire end-to-end path that the
packet takes.
- Router-to-Host. IPv6/IPv4 routers can tunnel IPv6 packets to
their final destination IPv6/IPv4 host. This tunnel spans only
the last segment of the end-to-end path.
Configured tunneling can be used in all of the above cases, but it is
most likely to be used router-to-router due to the need to explicitly
configure the tunneling endpoints.
The underlying mechanisms for tunneling are:
- The entry node of the tunnel (the encapsulator) creates an
encapsulating IPv4 header and transmits the encapsulated packet.
- The exit node of the tunnel (the decapsulator) receives the
encapsulated packet, reassembles the packet if needed, removes the
IPv4 header, and processes the received IPv6 packet.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
- The encapsulator may need to maintain soft-state information for
each tunnel recording such parameters as the MTU of the tunnel in
order to process IPv6 packets forwarded into the tunnel.
In configured tunneling, the tunnel endpoint addresses are determined
in the encapsulator from configuration information stored for each
tunnel. When an IPv6 packet is transmitted over a tunnel, the
destination and source addresses for the encapsulating IPv4 header
are set as described in Section 3.5.
The determination of which packets to tunnel is usually made by
routing information on the encapsulator. This is usually done via a
routing table, which directs packets based on their destination
address using the prefix mask and match technique.
The decapsulator matches the received protocol-41 packets to the
tunnels it has configured, and allows only the packets in which IPv4
source addresses match the tunnels configured on the decapsulator.
Therefore, the operator must ensure that the tunnel's IPv4 address
configuration is the same both at the encapsulator and the
decapsulator.
3.1. Encapsulation
The encapsulation of an IPv6 datagram in IPv4 is shown below:
+-------------+
| IPv4 |
| Header |
+-------------+ +-------------+
| IPv6 | | IPv6 |
| Header | | Header |
+-------------+ +-------------+
| Transport | | Transport |
| Layer | ===> | Layer |
| Header | | Header |
+-------------+ +-------------+
| | | |
~ Data ~ ~ Data ~
| | | |
+-------------+ +-------------+
Encapsulating IPv6 in IPv4
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
In addition to adding an IPv4 header, the encapsulator also has to
handle some more complex issues:
- Determine when to fragment and when to report an ICMPv6 "packet
too big" error back to the source.
- How to reflect ICMPv4 errors from routers along the tunnel path
back to the source as ICMPv6 errors.
Those issues are discussed in the following sections.
3.2. Tunnel MTU and Fragmentation
Naively, the encapsulator could view encapsulation as IPv6 using IPv4
as a link layer with a very large MTU (65535-20 bytes at most; 20
bytes "extra" are needed for the encapsulating IPv4 header). The
encapsulator would only need to report ICMPv6 "packet too big" errors
back to the source for packets that exceed this MTU. However, such a
scheme would be inefficient or non-interoperable for three reasons
and therefore MUST NOT be used:
1) It would result in more fragmentation than needed. IPv4 layer
fragmentation should be avoided due to the performance problems
caused by the loss unit being smaller than the retransmission unit
[KM97].
2) Any IPv4 fragmentation occurring inside the tunnel, i.e., between
the encapsulator and the decapsulator, would have to be
reassembled at the tunnel endpoint. For tunnels that terminate at
a router, this would require additional memory and other resources
to reassemble the IPv4 fragments into a complete IPv6 packet
before that packet could be forwarded.
3) The encapsulator has no way of knowing that the decapsulator is
able to defragment such IPv4 packets (see Section 3.6 for
details), and has no way of knowing that the decapsulator is able
to handle such a large IPv6 Maximum Receive Unit (MRU).
Hence, the encapsulator MUST NOT treat the tunnel as an interface
with an MTU of 64 kilobytes, but instead either use the fixed static
MTU or OPTIONAL dynamic MTU determination based on the IPv4 path MTU
to the tunnel endpoint.
If both the mechanisms are implemented, the decision of which to use
SHOULD be configurable on a per-tunnel endpoint basis.
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RFC 4213 Basic IPv6 Transition Mechanisms October 20053.2.1. Static Tunnel MTU
A node using static tunnel MTU treats the tunnel interface as having
a fixed-interface MTU. By default, the MTU MUST be between 1280 and
1480 bytes (inclusive), but it SHOULD be 1280 bytes. If the default
is not 1280 bytes, the implementation MUST have a configuration knob
that can be used to change the MTU value.
A node must be able to accept a fragmented IPv6 packet that, after
reassembly, is as large as 1500 octets [RFC2460]. This memo also
includes requirements (see Section 3.6) for the amount of IPv4
reassembly and IPv6 MRU that MUST be supported by all the
decapsulators. These ensure correct interoperability with any fixed
MTUs between 1280 and 1480 bytes.
A larger fixed MTU than supported by these requirements must not be
configured unless it has been administratively ensured that the
decapsulator can reassemble or receive packets of that size.
The selection of a good tunnel MTU depends on many factors, at least:
- Whether the IPv4 protocol-41 packets will be transported over
media that may have a lower path MTU (e.g., IPv4 Virtual Private
Networks); then picking too high a value might lead to IPv4
fragmentation.
- Whether the tunnel is used to transport IPv6 tunneled packets
(e.g., a mobile node with an IPv6-in-IPv4 configured tunnel, and
an IPv6-in-IPv6 tunnel interface); then picking too low a value
might lead to IPv6 fragmentation.
If layered encapsulation is believed to be present, it may be prudent
to consider supporting dynamic MTU determination instead as it is
able to minimize fragmentation and optimize packet sizes.
When using the static tunnel MTU, the Don't Fragment bit MUST NOT be
set in the encapsulating IPv4 header. As a result, the encapsulator
should not receive any ICMPv4 "packet too big" messages as a result
of the packets it has encapsulated.
3.2.2. Dynamic Tunnel MTU
The dynamic MTU determination is OPTIONAL. However, if it is
implemented, it SHOULD have the behavior described in this document.
The fragmentation inside the tunnel can be reduced to a minimum by
having the encapsulator track the IPv4 path MTU across the tunnel,
using the IPv4 Path MTU Discovery Protocol [RFC1191] and recording
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
the resulting path MTU. The IPv6 layer in the encapsulator can then
view a tunnel as a link layer with an MTU equal to the IPv4 path MTU,
minus the size of the encapsulating IPv4 header.
Note that this does not eliminate IPv4 fragmentation in the case when
the IPv4 path MTU would result in an IPv6 MTU less than 1280 bytes.
(Any link layer used by IPv6 has to have an MTU of at least 1280
bytes [RFC2460].) In this case, the IPv6 layer has to "see" a link
layer with an MTU of 1280 bytes and the encapsulator has to use IPv4
fragmentation in order to forward the 1280 byte IPv6 packets.
The encapsulator SHOULD employ the following algorithm to determine
when to forward an IPv6 packet that is larger than the tunnel's path
MTU using IPv4 fragmentation, and when to return an ICMPv6 "packet
too big" message per [RFC1981]:
if (IPv4 path MTU - 20) is less than 1280
if packet is larger than 1280 bytes
Send ICMPv6 "packet too big" with MTU = 1280.
Drop packet.
else
Encapsulate but do not set the Don't Fragment
flag in the IPv4 header. The resulting IPv4
packet might be fragmented by the IPv4 layer
on the encapsulator or by some router along
the IPv4 path.
endif
else
if packet is larger than (IPv4 path MTU - 20)
Send ICMPv6 "packet too big" with
MTU = (IPv4 path MTU - 20).
Drop packet.
else
Encapsulate and set the Don't Fragment flag
in the IPv4 header.
endif
endif
Encapsulators that have a large number of tunnels may choose between
dynamic versus static tunnel MTUs on a per-tunnel endpoint basis. In
cases where the number of tunnels that any one node is using is
large, it is helpful to observe that this state information can be
cached and discarded when not in use.
Note that using dynamic tunnel MTU is subject to IPv4 path MTU
blackholes should the ICMPv4 "packet too big" messages be dropped by
firewalls or not generated by the routers [RFC1435, RFC2923].
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RFC 4213 Basic IPv6 Transition Mechanisms October 20053.3. Hop Limit
IPv6-over-IPv4 tunnels are modeled as "single-hop" from the IPv6
perspective. The tunnel is opaque to users of the network, and it is
not detectable by network diagnostic tools such as traceroute.
The single-hop model is implemented by having the encapsulators and
decapsulators process the IPv6 hop limit field as they would if they
were forwarding a packet on to any other datalink. That is, they
decrement the hop limit by 1 when forwarding an IPv6 packet. (The
originating node and final destination do not decrement the hop
limit.)
The TTL of the encapsulating IPv4 header is selected in an
implementation-dependent manner. The current suggested value is
published in the "Assigned Numbers" RFC [RFC3232][ASSIGNED].
Implementations MAY provide a mechanism to allow the administrator to
configure the IPv4 TTL as the IP Tunnel MIB [RFC4087].
3.4. Handling ICMPv4 Errors
In response to encapsulated packets it has sent into the tunnel, the
encapsulator might receive ICMPv4 error messages from IPv4 routers
inside the tunnel. These packets are addressed to the encapsulator
because it is the IPv4 source of the encapsulated packet.
ICMPv4 error handling is only applicable to dynamic MTU
determination, even though the functions could be used with static
MTU tunnels as well.
The ICMPv4 "packet too big" error messages are handled according to
IPv4 Path MTU Discovery [RFC1191] and the resulting path MTU is
recorded in the IPv4 layer. The recorded path MTU is used by IPv6 to
determine if an ICMPv6 "packet too big" error has to be generated as
described in Section 3.2.2.
The handling of other types of ICMPv4 error messages depends on how
much information is available from the encapsulated packet that
caused the error.
Many older IPv4 routers return only 8 bytes of data beyond the IPv4
header of the packet in error, which is not enough to include the
address fields of the IPv6 header. More modern IPv4 routers are
likely to return enough data beyond the IPv4 header to include the
entire IPv6 header and possibly even the data beyond that. See
[RFC1812].
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
If sufficient data bytes from the offending packet are available, the
encapsulator MAY extract the encapsulated IPv6 packet and use it to
generate an ICMPv6 message directed back to the originating IPv6
node, as shown below:
+--------------+
| IPv4 Header |
| dst = encaps |
| node |
+--------------+
| ICMPv4 |
| Header |
- - +--------------+
| IPv4 Header |
| src = encaps |
IPv4 | node |
+--------------+ - -
Packet | IPv6 |
| Header | Original IPv6
in +--------------+ Packet -
| Transport | Can be used to
Error | Header | generate an
+--------------+ ICMPv6
| | error message
~ Data ~ back to the source.
| |
- - +--------------+ - -
ICMPv4 Error Message Returned to Encapsulating Node
When receiving ICMPv4 errors as above and the errors are not "packet
too big", it would be useful to log the error as an error related to
the tunnel. Also, if sufficient headers are available, then the
originating node MAY send an ICMPv6 error of type "unreachable" with
code "address unreachable" to the IPv6 source. (The "address
unreachable" code is appropriate since, from the perspective of IPv6,
the tunnel is a link and that code is used for link-specific errors
[RFC2463]).
Note that when the IPv4 path MTU is exceeded, and sufficient bytes of
payload associated with the ICMPv4 errors are not available, or
ICMPv4 errors do not cause the generation of ICMPv6 errors in case
there is enough payload, there will be at least two packet drops
instead of at least one (the case of a single layer of MTU
discovery). Consider a case where an IPv6 host is connected to an
IPv4/IPv6 router, which is connected to a network where an ICMPv4
error about too big packet size is generated. First, the router
needs to learn the tunnel (IPv4) MTU that causes at least one packet
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
loss, and then the host needs to learn the (IPv6) MTU from the router
that causes at least one packet loss. Still, in all cases there can
be more than one packet loss if there are multiple large packets in
flight at the same time.
3.5. IPv4 Header Construction
When encapsulating an IPv6 packet in an IPv4 datagram, the IPv4
header fields are set as follows:
Version:
4
IP Header Length in 32-bit words:
5 (There are no IPv4 options in the encapsulating header.)
Type of Service:
0 unless otherwise specified. (See [RFC2983] and [RFC3168]
Section 9.1 for issues relating to the Type-of-Service byte and
tunneling.)
Total Length:
Payload length from IPv6 header plus length of IPv6 and IPv4
headers (i.e., IPv6 payload length plus a constant 60 bytes).
Identification:
Generated uniquely as for any IPv4 packet transmitted by the
system.
Flags:
Set the Don't Fragment (DF) flag as specified in Section 3.2.
Set the More Fragments (MF) bit as necessary if fragmenting.
Fragment Offset:
Set as necessary if fragmenting.
Time to Live:
Set in an implementation-specific manner, as described in
Section 3.3.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
Protocol:
41 (Assigned payload type number for IPv6).
Header Checksum:
Calculate the checksum of the IPv4 header [RFC791].
Source Address:
An IPv4 address of the encapsulator: either configured by the
administrator or an address of the outgoing interface.
Destination Address:
IPv4 address of the tunnel endpoint.
When encapsulating the packets, the node must ensure that it will use
the correct source address so that the packets are acceptable to the
decapsulator as described in Section 3.6. Configuring the source
address is appropriate particularly in cases in which automatic
selection of source address may produce different results in a
certain period of time. This is often the case with multiple
addresses, and multiple interfaces, or when routes may change
frequently. Therefore, it SHOULD be possible to administratively
specify the source address of a tunnel.
3.6. Decapsulation
When an IPv6/IPv4 host or a router receives an IPv4 datagram that is
addressed to one of its own IPv4 addresses or a joined multicast
group address, and the value of the protocol field is 41, the packet
is potentially a tunnel packet and needs to be verified to belong to
one of the configured tunnel interfaces (by checking
source/destination addresses), reassembled (if fragmented at the IPv4
level), and have the IPv4 header removed and the resulting IPv6
datagram be submitted to the IPv6 layer code on the node.
The decapsulator MUST verify that the tunnel source address is
correct before further processing packets, to mitigate the problems
with address spoofing (see Section 4). This check also applies to
packets that are delivered to transport protocols on the
decapsulator. This is done by verifying that the source address is
the IPv4 address of the encapsulator, as configured on the
decapsulator. Packets for which the IPv4 source address does not
match MUST be discarded and an ICMP message SHOULD NOT be generated;
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
however, if the implementation normally sends an ICMP message when
receiving an unknown protocol packet, such an error message MAY be
sent (e.g., ICMPv4 Protocol 41 Unreachable).
A side effect of this address verification is that the node will
silently discard packets with a wrong source address and packets that
were received by the node but not directly addressed to it (e.g.,
broadcast addresses).
Independent of any other forms of IPv4 ingress filtering the
administrator of the node may have configured, the implementation MAY
perform ingress filtering, i.e., check that the packet is arriving
from the interface in the direction of the route toward the tunnel
end-point, similar to a Strict Reverse Path Forwarding (RPF) check
[RFC3704]. As this may cause problems on tunnels that are routed
through multiple links, it is RECOMMENDED that this check, if done,
is disabled by default. The packets caught by this check SHOULD be
discarded; an ICMP message SHOULD NOT be generated by default.
The decapsulator MUST be capable of having, on the tunnel interfaces,
an IPv6 MRU of at least the maximum of 1500 bytes and the largest
(IPv6) interface MTU on the decapsulator.
The decapsulator MUST be capable of reassembling an IPv4 packet that
is (after the reassembly) the maximum of 1500 bytes and the largest
(IPv4) interface MTU on the decapsulator. The 1500-byte number is a
result of encapsulators that use the static MTU scheme in Section3.2.1, while encapsulators that use the dynamic scheme in Section3.2.2 can cause up to the largest interface MTU on the decapsulator
to be received. (Note that it is strictly the interface MTU on the
last IPv4 router *before* the decapsulator that matters, but for most
links the MTU is the same between all neighbors.)
This reassembly limit allows dynamic tunnel MTU determination by the
encapsulator to take advantage of larger IPv4 path MTUs. An
implementation MAY have a configuration knob that can be used to set
a larger value of the tunnel reassembly buffers than the above
number, but it MUST NOT be set below the above number.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
In addition, the node should be configured to perform ingress
filtering [RFC2827][RFC3704] on the IPv6 source address, similar to
on any of its interfaces, e.g.:
1) if the tunnel is toward the Internet, the node should be
configured to check that the site's IPv6 prefixes are not used as
the source addresses, or
2) if the tunnel is toward an edge network, the node should be
configured to check that the source address belongs to that edge
network.
The prefix lists in the former typically need to be manually
configured; the latter could be verified automatically, e.g., by
using a strict unicast RPF check, as long as an interface can be
designated to be toward an edge.
It is RECOMMENDED that the implementations provide a single knob to
make it easier to for the administrators to enable strict ingress
filtering toward edge networks.
3.7. Link-Local Addresses
The configured tunnels are IPv6 interfaces (over the IPv4 "link
layer") and thus MUST have link-local addresses. The link-local
addresses are used by, e.g., routing protocols operating over the
tunnels.
The interface identifier [RFC3513] for such an interface may be based
on the 32-bit IPv4 address of an underlying interface, or formed
using some other means, as long as it is unique from the other tunnel
endpoint with a reasonably high probability.
Note that it may be desirable to form the link-local address in a
fashion that minimizes the probability and the effect of having to
renumber the link-local address in the event of a topology or
hardware change.
If an IPv4 address is used for forming the IPv6 link-local address,
the interface identifier is the IPv4 address, prepended by zeros.
Note that the "Universal/Local" bit is zero, indicating that the
interface identifier is not globally unique. The link-local address
is formed by appending the interface identifier to the prefix
FE80::/64.
When the host has more than one IPv4 address in use on the physical
interface concerned, a choice of one of these IPv4 addresses is made
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
by the administrator or the implementation when forming the link-
local address.
+-------+-------+-------+-------+-------+-------+------+------+
| FE 80 00 00 00 00 00 00 |
+-------+-------+-------+-------+-------+-------+------+------+
| 00 00 00 00 | IPv4 Address |
+-------+-------+-------+-------+-------+-------+------+------+
3.8. Neighbor Discovery over Tunnels
Configured tunnel implementations MUST at least accept and respond to
the probe packets used by Neighbor Unreachability Detection (NUD)
[RFC2461]. The implementations SHOULD also send NUD probe packets to
detect when the configured tunnel fails at which point the
implementation can use an alternate path to reach the destination.
Note that Neighbor Discovery allows that the sending of NUD probes be
omitted for router-to-router links if the routing protocol tracks
bidirectional reachability.
For the purposes of Neighbor Discovery, the configured tunnels
specified in this document are assumed to NOT have a link-layer
address, even though the link-layer (IPv4) does have an address.
This means that:
- the sender of Neighbor Discovery packets SHOULD NOT include Source
Link Layer Address options or Target Link Layer Address options on
the tunnel link.
- the receiver MUST, while otherwise processing the Neighbor
Discovery packet, silently ignore the content of any Source Link
Layer Address options or Target Link Layer Address options
received on the tunnel link.
Not using link-layer address options is consistent with how Neighbor
Discovery is used on other point-to-point links.
4. Threat Related to Source Address Spoofing
The specification above contains rules that apply tunnel source
address verification in particular and ingress filtering
[RFC2827][RFC3704] in general to packets before they are
decapsulated. When IP-in-IP tunneling (independent of IP versions)
is used, it is important that this not be used to bypass any ingress
filtering in use for non-tunneled packets. Thus, the rules in this
document are derived based on should ingress filtering be used for
IPv4 and IPv6, the use of tunneling should not provide an easy way to
circumvent the filtering.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
In this case, without specific ingress filtering checks in the
decapsulator, it would be possible for an attacker to inject a packet
with:
- Outer IPv4 source: real IPv4 address of attacker
- Outer IPv4 destination: IPv4 address of decapsulator
- Inner IPv6 source: Alice, which is either the decapsulator or a
node close to it
- Inner IPv6 destination: Bob
Even if all IPv4 routers between the attacker and the decapsulator
implement IPv4 ingress filtering, and all IPv6 routers between the
decapsulator and Bob implement IPv6 ingress filtering, the above
spoofed packets will not be filtered out. As a result, Bob will
receive a packet that looks like it was sent from Alice even though
the sender was some unrelated node.
The solution to this is to have the decapsulator accept only
encapsulated packets from the explicitly configured source address
(i.e., the other end of the tunnel) as specified in Section 3.6.
While this does not provide complete protection in the case ingress
filtering has not been deployed, it does provide a significant
increase in security. The issue and the remainder threats are
discussed at more length in Security Considerations.
5. Security Considerations
Generic security considerations of using IPv6 are discussed in a
separate document [V6SEC].
An implementation of tunneling needs to be aware that although a
tunnel is a link (as defined in [RFC2460]), the threat model for a
tunnel might be rather different than for other links, since the
tunnel potentially includes all of the Internet.
Several mechanisms (e.g., Neighbor Discovery) depend on Hop Count
being 255 and/or the addresses being link local for ensuring that a
packet originated on-link, in a semi-trusted environment. Tunnels
are more vulnerable to a breach of this assumption than physical
links, as an attacker anywhere in the Internet can send an IPv6-in-
IPv4 packet to the tunnel decapsulator, causing injection of an
encapsulted IPv6 packet to the configured tunnel interface unless the
decapsulation checks are able to discard packets injected in such a
manner.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
Therefore, this memo specifies that the decapsulators make these
steps (as described in Section 3.6) to mitigate this threat:
- IPv4 source address of the packet MUST be the same as configured
for the tunnel end-point;
- Independent of any IPv4 ingress filtering the administrator may
have configured, the implementation MAY perform IPv4 ingress
filtering to check that the IPv4 packets are received from an
expected interface (but as this may cause some problems, it may be
disabled by default);
- IPv6 packets with several, obviously invalid IPv6 source addresses
received from the tunnel MUST be discarded (see Section 3.6 for
details); and
- IPv6 ingress filtering should be performed (typically requiring
configuration from the operator), to check that the tunneled IPv6
packets are received from an expected interface.
Especially the first verification is vital: to avoid this check, the
attacker must be able to know the source of the tunnel (ranging from
difficult to predictable) and be able to spoof it (easier).
If the remainder threats of tunnel source verification are considered
to be significant, a tunneling scheme with authentication should be
used instead, e.g., IPsec [RFC2401] (preferable) or Generic Routing
Encapsulation with a pre-configured secret key [RFC2890]. As the
configured tunnels are set up more or less manually, setting up the
keying material is probably not a problem. However, setting up
secure IPsec IPv6-in-IPv4 tunnels is described in another document
[V64IPSEC].
If the tunneling is done inside an administrative domain, proper
ingress filtering at the edge of the domain can also eliminate the
threat from outside of the domain. Therefore, shorter tunnels are
preferable to longer ones, possibly spanning the whole Internet.
In addition, an implementation MUST treat interfaces to different
links as separate, e.g., to ensure that Neighbor Discovery packets
arriving on one link do not affect other links. This is especially
important for tunnel links.
When dropping packets due to failing to match the allowed IPv4 source
addresses for a tunnel the node should not "acknowledge" the
existence of a tunnel, otherwise this could be used to probe the
acceptable tunnel endpoint addresses. For that reason, the
specification says that such packets MUST be discarded, and an ICMP
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
error message SHOULD NOT be generated, unless the implementation
normally sends ICMP destination unreachable messages for unknown
protocols; in such a case, the same code MAY be sent. As should be
obvious, not returning the same ICMP code if an error is returned for
other protocols may hint that the IPv6 stack (or the protocol 41
tunneling processing) has been enabled -- the behaviour should be
consistent on how the implementation otherwise behaves to be
transparent to probing.
6. Acknowledgements
We would like to thank the members of the IPv6 working group, the
Next Generation Transition (ngtrans) working group, and the v6ops
working group for their many contributions and extensive review of
this document. Special thanks are due to (in alphabetical order) Jim
Bound, Ross Callon, Tim Chown, Alex Conta, Bob Hinden, Bill Manning,
John Moy, Mohan Parthasarathy, Chirayu Patel, Pekka Savola, and Fred
Templin for many helpful suggestions. Pekka Savola helped in editing
the final revisions of the specification.
7. References7.1. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2463] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
7.2. Informative References
[ASSIGNED] IANA, "Assigned numbers online database",
http://www.iana.org/numbers.htmlNordmark & Gilligan Standards Track [Page 21]

RFC 4213 Basic IPv6 Transition Mechanisms October 2005
- Split into normative and non-normative references and other
reference cleanup.
- Dropped "or equal" in if (IPv4 path MTU - 20) is less than or
equal to 1280.
- Dropped this: However, IPv6 may be used in some environments where
interoperability with IPv4 is not required. IPv6 nodes that are
designed to be used in such environments need not use or even
implement these mechanisms.
- Described Static MTU and Dynamic MTU cases separately; clarified
that the dynamic path MTU mechanism is OPTIONAL but if it is
implemented it should follow the rules in section 3.2.2.
- Specified Static MTU to default to a MTU of 1280 to 1480 bytes,
and that this may be configurable. Discussed the issues with
using Static MTU at more length.
- Specified minimal rules for IPv4 reassembly and IPv6 MRU to
enhance interoperability and to minimize blacholes.
- Restated the "currently underway" language about Type-of-Service,
and loosely point at [RFC2983] and [RFC3168].
- Fixed reference to Assigned Numbers to be to online version (with
proper pointer to "Assigned Numbers is obsolete" RFC).
- Clarified text about ingress filtering e.g., that it applies to
packet delivered to transport protocols on the decapsulator as
well as packets being forwarded by the decapsulator, and how the
decapsulator's checks help when IPv4 and IPv6 ingress filtering is
in place.
- Removed unidirectional tunneling; assume all tunnels are
bidirectional, between endpoint addresses (not nodes).
- Removed the guidelines for advertising addresses in DNS as
slightly out of scope, referring to another document for the
details.
- Removed the SHOULD requirement that the link-local addresses
should be formed based on IPv4 addresses.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
- Added a SHOULD for implementing a knob to be able to set the
source address of the tunnel, and add discussion why this is
useful.
- Added stronger wording for source address checks: both IPv4 and
IPv6 source addresses MUST be checked, and RPF-like ingress
filtering is optional.
- Rewrote security considerations to be more precise about the
threats of tunneling.
- Added a note about considering using TTL=255 when encapsulating.
- Added more discussion in Section 3.2 why using an "infinite" IPv6
MTU leads to likely interoperability problems.
- Added an explicit requirement that if both MTU determination
methods are used, choosing one should be possible on a per-tunnel
basis.
- Clarified that ICMPv4 error handling is only applicable to dynamic
MTU determination.
- Removed/clarified DNS record filtering; an API is a SHOULD and if
it does not exist, MUST NOT filter anything. Decree ordering out
of scope, but refer to RFC3484.
- Add a note that the destination IPv4 address could also be a
multicast address.
- Make it RECOMMENDED to provide a toggle to perform strict ingress
filtering on an interface.
- Generalize the text on the data in ICMPv4 messages.
- Made a lot of miscellaneous editorial cleanups.
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RFC 4213 Basic IPv6 Transition Mechanisms October 2005
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Nordmark & Gilligan Standards Track [Page 27]